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Little made a point of underlining Labour's stability, saying the party's front bench would remain unchanged until the election. So often the party whose internal divisions are on display, Labour appeared to be relishing the prospect of a potentially messy National Party leadership contest.

Standing next to him, Labour's deputy leader Annette King struggled to contain her grin.

"I think politics is a very uncertain game," King told reporters. "I've been around long enough to ... expect the unexpected."

Little, whose party is coming off a landslide byelection win in Mt Roskill, spent most of his press conference applauding Key's record. Asked about his legacy, Little pointed to Key's leadership following the global financial crisis and several natural disasters.

"The role of Prime Minister is a tough gig at any time," Little said.

"It is hugely challenging, not only in the role but for family members.

"I want to pay tribute to the contribution that John Key has made to New Zealand in his time as Prime Minister."

New Zealand First leader Winston Peters was less generous, saying that Key's reasons for standing down were "not credible". He said Key was leaving because the economy was no longer healthy and his Government was no longer able to "muddy the waters".

Meanwhile, National's support partners were questioning where they stood yesterday.

"The ball game has changed," said United Future leader Peter Dunne, who has benefited from National endorsements in the past two elections.

"It is not clear how. But Key is a dominant figure. He is not around to be around the scene. I think there's a while lot of questions but it's far too soon to attempt to answer them."

Maori Party co-leaders Te Ururoa Flavell and Marama Fox said they were confident the new Prime Minister would continue a "mana enhancing" relationship with the Maori Party.